The present invention relates to a surface grinding machine for grinding the back surface of a wafer of a single crystal III-V group compound semiconductor on which elements have been fabricated.
The III-V group compound semiconductors include GaAs, InSb, InP, GaP, GaSb, etc. These compound semiconductors have a common disadvantage in that they are soft and fragile compared with silicon.
A single crystal of a compound semiconductor is prepared by the Liquid Encapsulated Czochralski (LEC) or Horizontal Bridgman (HB) method. The single crystal compound semiconductor is ground into a columnar shape with the orientation flat (OF) or IF.
The columnar single crystal ingot is cut into thin discs (or square plates) called an as-cut-wafer.
In order to make the thickness even, the as-cut-wafers are subjected to both or one side lapping, and both sides or one side of each is subjected to mirror polishing. In the meantime, the wafer is subjected to etching several times to remove the layer changed in property by working, and often subjected to beveling to round the peripheral edge. The product thus obtained is called a mirror wafer.
The present invention is not concerned with grinding in the process of making the mirror wafer from the as-cut-wafer.
Various of elements are fabricated on the mirror wafer by repeating wafer processes. The elements may be light emitting elements, integrated circuits of high-speed logic elements, light receiving elements, or elements for detecting infrared rays etc. Depending on the intended purpose, varieties of wafer processes such as epitaxial growth, ion implantation, etching, vapor deposition, or insulating film formation, are used.
The present invention is intended for a wafer on which elements have been fabricated.
The wafer with elements is roughly 620 .mu.m.about.700 .mu.m thick for a 3 inch diameter since the thickness of the mirror wafer is just about that size. When elements are fabricated, the thickness of a layer slightly changes because of epitaxial growth or the like by several .mu.m to the utmost so that the thickness of the wafer almost equals to that of the mirror wafer.
The wafer is a little thick since the mechanical strength is required when elements are fabricated. If the wafer is thinner than the above value, the handling of the wafer is difficult.
In case that a semiconductor element is fabricated, the wafer is only used as a substrate and its surface of only several .mu.m thick is necessary for the fabrication. The other part of the wafer is required to simply impart it mechanical strength.
Moreover, these elements generate heat when they are actually operated. The larger the degree of integration of an integrated circuit becomes, the greater the heat generation becomes. Also in case of a light emitting element, the problem of heat generation is serious because a large forward current is passed therethrough.
Moreover, elements employing a single crystal compound semiconductor wafer have characteristics of high-speed operation. In order to operate an element at high speed, a large current must generally be kept flowing and the consumption of current becomes greater. Accordingly, an element of GaAs etc. poses a serious problem in view of heat generation as compared with a silicon semiconductor element.
An additional disadvantage is that the thermal conductivity of the compound semiconductor is lower than that of silicon. The heat generated by the elements mostly passes through a chip and escapes from the back of the chip into a package.
Also the package is designed to accelerate heat radiation. For example, the package is made by laminating thin ceramic plates of Al.sub.2 O.sub.3 and the like, and the part contacted with an IC chip is made of a metal plate.
There is also a problem of the efficiency of heat radiation within the chip due to the heat transfer from the surface to the back thereof. The heat radiation is accelerated by merely reducing the thickness of the semiconductor chip. Consequently, the back of the wafer is ground to reduce its thickness after elements have been fabricated.
Also in an Si semiconductor, the back thereof is ground to reduce its thickness in case that a great deal of thermal generation occurs. Since the thermal conductivity of silicon is excellent, it is sufficient to reduce the thickness to about 400 .mu.m.
In case of the Si semiconductor, lapping is employed to grind the back thereof. The lapping employed in this stage is different in purpose from that employed in the process of making the mirror wafer from the as-cut-wafer. However, the technique is similar to each other. The surface of the wafer is secured to a suitable pressure disc. By turning the pressure disc and contacting the disc to a platen while supplying an abrasive, the back of the wafer is lapped by the rotation of the platen and the pressure disc. The abrasive contains a large amount of abrasive grains. The back of the wafer physically contacts the abrasive grains and is shaved.
Although lapping is usable to make the wafer 400 .mu.m thick, it is wet processing and therefore not necessarily a good method. That is, the processing time including pre- and after-processing is lengthy. As the abrasive grains are used, they may be embedded in the surface of the wafer on which elements are fabricated and thus must be washed off. The layer changed in property by lapping is large. Also, there is a problem of dealing with a large amount of waste liquid. Moreover, automation cannot be attained due to the batch processing. As set forth above, there are a number of disadvantages in the lapping method for shaving the back of the wafer on which elements are not fabricated.
Accordingly, grinding the back of the Si wafer by means of a diamond wheel was earnestly demanded.
In response to such a demand, the present inventors have succeeded in realizing a method of grinding the back of the Si wafer by a diamond wheel. The method uses a surface grinding machine as disclosed in Japanese Unexamined Published Application No. 95866/86 (laid open on May 14, 1986).
The aforesaid method has such advantages that fixed abrasive grains are used instead of free abrasive grains, processing time is short, and automation can be attained.
Such grinding the back of the wafer by means of the diamond wheel is simply called back grinding.
Due to the success of the present inventors, the back grinding is being used instead of lapping in order to reduce the thickness of the Si wafer. Although lapping is mainly used at present, back grinding seems to be mainly used in the future.
The above description is intended to show the need of making a wafer thinner and changes of methods used in processing the silicon wafer.
In case of the III-V group compound, there exists a decisive difficulty that it is fragile compared with the Si wafer.
Moreover, the thermal conductivity of the III-V group compound is lower than that of the Si wafer and, because the former compound is operated at high speed, it generates a great deal of heat. For that reason, the III-V group compound must be made as thin as up to 200 .mu.m, whereas it is only necessary to make the Si wafer as thin as up to 400 .mu.m. The III-V group compound is more disadvantageous as compared with the Si wafer.
Accordingly, lapping has mainly been employed for making the compound semiconductor wafer thin. Because of lapping, free abrasive grains are used. The back of the wafer is shaved without difficulty by the liquid containing the free abrasive grains. Consequently, the wafer is seldom broken or chipped off even if it is made as thin as 200 .mu.m by grinding.
Thus, the most suitable way of making the compound semiconductor wafer thin was lapping and even now lapping is being used.
As set forth above, however, lapping is quite an inefficient method since pre- and post-processing is troublesome. Further, it has such disadvantages that the wafer must thoroughly be washed in order not to remain the abrasive gains, a large amount of waste liquid is produced, dealing with waste liquid is a difficult problem, and it is not suitable for automatic operation as it cannot be performed continuously.
There is a strong demand for thinning a compound semiconductor wafer by a diamond wheel. Although such a method has already been put to practical use for grinding silicon wafer, it can not always be applicable to the compound semiconductor wafer. Silicon is firm and hardly breakable. On the other hand, cleavage easily arises in compound semiconductors such as GaAs by a small force and are thus fragile and breakable. For that reason, back grinding by means of the diamond wheel was deemed impossible.
The compound semiconductor wafer is likely to be broken when physically contacting with the wheel. It is often broken because it is to be shaved as thin as about half of that of the Si wafer notwithstanding its fragility compared with the Si wafer.
Even though the compound semiconductor wafer is not broken, the surface thereof will be torn off along its cleavage plane. In other words, there are produced a number of cavities in the surface thereof. This is because the abrasive grains fixed to the wheel scrape the soft portion of the surface thereof locally.
If the surface is torn off, the back of the wafer does not become a mirror surface. If the back thereof is not a mirror surface, the chip will not smoothly contact with the package when the chip is die-bonded to the package and this causes the thermal resistance to inconveniently increase.
Namely, grinding the back of the fragile compound semiconductor wafer is very difficult compared with the case of grinding the Si wafer.